After talks in Saudi Arabia's summer capital Jeddah, Secretary of State John Kerry won backing from 10 Arab countries - Egypt, Iraq, Jordan, Lebanon and six Gulf states including rich rivals Saudi Arabia and Qatar - for a coalition to fight the Sunni militants that have seized swathes of Iraq and Syria.
"Arab nations play a critical role in that coalition, the leading role really across all lines of effort: military support, humanitarian aid, our work to stop the flow of illegal funds," Kerry told a news conference.
Non-Arab Sunni power Turkey also attended the Jeddah talks but two other major regional players - Shi'ite Iran and Syria itself - were excluded, a sign of the difficulty of building a coalition across the Middle East's sectarian battle lines.
The Arab states agreed in a communique to do more to stop the flow of funds and fighters to Islamic State and help rebuild communities "brutalized" by the group.
"The participating states agreed to do their share in the comprehensive fight against ISIS, including ... as appropriate, joining in the many aspects of a coordinated military campaign against ISIS," they said, using the acronym for Islamic State in Iraq and Syria, a former name for the group.
Kerry met the Arab leaders to drum up support a day after Obama announced his plans to strike fighters in Iraq and Syria.
US officials said Kerry also sought permission to make more use of bases in the region and fly more warplanes overhead, issues that were not mentioned in the communique. Kerry said none of the countries in the coalition would send ground troops.
In a hopeful sign of outreach across the sectarian divide that has spread war across the Middle East and fed Islamic State's militancy, Sunni Saudi Arabia said it might open an embassy in Shi'ite-ruled Iraq after decades of suspicion.
The Saudis, who support other Sunni armed movements in Syria but consider Islamic State a terrorist group, have also promised to help Obama's campaign by providing training camps for moderate Syrian Sunni fighters.
But Iran, the main Shi'ite power in the Middle East and supporter of Syria's President Bashar Assad, said it had severe reservations over the new US-led coalition, and doubted it would fight "the root causes of terrorism", which it blames squarely on Sunni Arab states like Saudi Arabia.
Obama announced his plans in a prime time address on Wednesday to build an alliance to root out Islamic State in both Syria and Iraq, plunging the United States into two conflicts in which nearly every country in the Middle East has a stake.
The region has been galvanized since June when Islamic State fighters, already in control of much of Syria, swept through northern Iraq, seizing cities, slaughtering prisoners, and proclaiming a "caliphate" that would rule over all Muslims.
The White House says the group is a threat to the West as well, attracting fighters from around the world who could return to carry out attacks at home.
Islamic State is a Sunni group that embraces a radical vision of a Middle East ruled along 7th century precepts. Its fighters are battling a Shi'ite-led government in Iraq and a Syrian government led by Assad, a follower of an offshoot of Shi'ite Islam. They are also fighting against more moderate Sunnis in Syria and against Kurds on both side of the frontier.
An alliance against Islamic State is bound to require cooperation from countries that consider each other enemies. Washington itself supports the Shi'ite-led government in Iraq but opposes Assad in Syria; it is allied to most Sunni Arab states while hostile to Iran.
A State Department official traveling with Kerry said the top US diplomat would the ask allies to make room for US military activity: "We may need enhanced basing and overflights ... there's going to be a meeting soon of defense ministers to work on these details."
Kerry would also urge regional television news outlets, particularly Qatari-owned Al Jazeera and Saudi-owned Al Arabiya, to air anti-extremist messages. Governments in the region would be urged to press mosques to preach against Islamic State.
"They need to get at the clerics because the clerics can get at the mosques in the neighborhood and they have to expose ISIS for what it is," the official told reporters.
Washington also wants more efforts to stop the flow of money to the group by tackling oil smuggling and cracking down on contributions from private donors, the official said.
Lebanese Foreign Minister Gebran Bassil told Reuters there was extensive discussion in Jeddah about whether the campaign should be broadened to include other Islamist groups, not just Islamic State. This is something that strongly anti-Islamist Gulf Arab states such as the United Arab Emirates had sought.
Despite attending the talks, Turkey was not mentioned in the communique, an omission attributed by a senior Turkish official to sensitivities about 46 Turkish hostages held by the fighters. Turkey will discuss the needs of the alliance with Kerry on a visit to Ankara he starts on Friday, the official said.
Isaiah 17 King James Version (KJV)
3 The fortress also shall cease from Ephraim, and the kingdom from Damascus, and the remnant of Syria: they shall be as the glory of the children of Israel, saith the Lord of hosts.
4 And in that day it shall come to pass, that the glory of Jacob shall be made thin, and the fatness of his flesh shall wax lean.
5 And it shall be as when the harvestman gathereth the corn, and reapeth the ears with his arm; and it shall be as he that gathereth ears in the valley of Rephaim.
6 Yet gleaning grapes shall be left in it, as the shaking of an olive tree, two or three berries in the top of the uppermost bough, four or five in the outmost fruitful branches thereof, saith the Lord God of Israel.
7 At that day shall a man look to his Maker, and his eyes shall have respect to the Holy One of Israel.
8 And he shall not look to the altars, the work of his hands, neither shall respect that which his fingers have made, either the groves, or the images.
9 In that day shall his strong cities be as a forsaken bough, and an uppermost branch, which they left because of the children of Israel: and there shall be desolation.
10 Because thou hast forgotten the God of thy salvation, and hast not been mindful of the rock of thy strength, therefore shalt thou plant pleasant plants, and shalt set it with strange slips:
11 In the day shalt thou make thy plant to grow, and in the morning shalt thou make thy seed to flourish: but the harvest shall be a heap in the day of grief and of desperate sorrow.
12 Woe to the multitude of many people, which make a noise like the noise of the seas; and to the rushing of nations, that make a rushing like the rushing of mighty waters!
13 The nations shall rush like the rushing of many waters: but God shall rebuke them, and they shall flee far off, and shall be chased as the chaff of the mountains before the wind, and like a rolling thing before the whirlwind.
14 And behold at eveningtide trouble; and before the morning he is not. This is the portion of them that spoil us, and the lot of them that rob us.
Psalm 83 King James Version (KJV)
3 They have taken crafty counsel against thy people, and consulted against thy hidden ones.
4 They have said, Come, and let us cut them off from being a nation; that the name of Israel may be no more in remembrance.
5 For they have consulted together with one consent: they are confederate against thee:
6 The tabernacles of Edom, and the Ishmaelites; of Moab, and the Hagarenes;
7 Gebal, and Ammon, and Amalek; the Philistines with the inhabitants of Tyre;
8 Assur also is joined with them: they have holpen the children of Lot. Selah.
9 Do unto them as unto the Midianites; as to Sisera, as to Jabin, at the brook of Kison:
10 Which perished at Endor: they became as dung for the earth.
11 Make their nobles like Oreb, and like Zeeb: yea, all their princes as Zebah, and as Zalmunna:
12 Who said, Let us take to ourselves the houses of God in possession.
13 O my God, make them like a wheel; as the stubble before the wind.
14 As the fire burneth a wood, and as the flame setteth the mountains on fire;
15 So persecute them with thy tempest, and make them afraid with thy storm.
16 Fill their faces with shame; that they may seek thy name, O Lord.
17 Let them be confounded and troubled for ever; yea, let them be put to shame, and perish:
18 That men may know that thou, whose name alone is Jehovah, art the most high over all the earth.